This invention relates generally to computed tomography (CT) imaging and more particularly, to image reconstruction using data collected by a CT system in a single slice helical scan.
In at least one known single slice CT system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the xe2x80x9cimaging planexe2x80x9d. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon a one row array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a xe2x80x9cviewxe2x80x9d. A xe2x80x9cscanxe2x80x9d of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and detector.
In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts that attenuation measurements from a scan into integers called xe2x80x9cCT numbersxe2x80x9d or xe2x80x9cHounsfield unitsxe2x80x9d, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
To reduce the total scan time, a xe2x80x9chelicalxe2x80x9d scan may be performed. To perform a xe2x80x9chelicalxe2x80x9d scan, the patient is moved while the data for the prescribed volume coverage is acquired. Such a system generates a single helix from a one fan beam helical scan. The helix mapped out by the fan beam yields projection data from which images in each prescribed slice may be reconstructed.
Reconstruction algorithms for helical scanning typically use helical weighting algorithms which weight the collected data as a function of view angle and detector channel index. Specifically, prior to filtered back projection, the data is weighted according to a helical weighting factor, which is a function of both the gantry angle and detector angle. Although the known algorithms generate compact slice profiles, some noticeable artifacts may be generated in the reconstructed image. Additional image quality issues with reconstructed images include a lack of xe2x80x9csharpnessxe2x80x9d in the image.
It would be desirable to provide an algorithm which facilitates reducing artifacts and increasing image xe2x80x9csharpnessxe2x80x9d. It also would be desirable to provide an algorithm which offers reasonable trade-offs between artifact reduction and slice profile in helical image reconstruction. It further would be desirable to provide such an algorithm which does not significantly increase the processing time.
These and other objects may be attained in a CT system configured to perform a single slice helical scan, which includes a projection domain z filtering algorithm that generates a modified weighting factor. More particularly, and with respect to generating the modified weighting factor, a helical reconstruction algorithm weighting factor is shifted in the view angle direction and averaged to generate the modified weighting factor. Examples of image reconstruction algorithms which may be utilized in reconstructing an image from data obtained in a helical scan are described in Crawford and King, xe2x80x9cComputed Tomography Scanning With Simultaneous Patient Translationxe2x80x9d, Med. Phys. 17(6), 967-982, 1990.
In one embodiment, the helical weighting factor is modified according to gantry angle (xcex2), detector angle (xcex3), and a filter kernel (h(i)) in accordance with the following:             W      f        ⁢          (              β        ,        γ            )        =            ∑              i        =                  -          n                            i        =        n              ⁢                  h        ⁢                  (          i          )                    ⁢              W        ⁢                  (                                    β              -                              i                ⁢                                  xe2x80x83                                ⁢                Δβ                                      ,            γ                    )                    
where:
xcex3 is the detector angle;
xcex2 is the gantry angle;
W(xcex2,xcex3) is the original weighting coefficient generated by the helical reconstruction algorithm;
xcex94xcex2 is the shift along the view angle direction; and
h(i) is the weighting applied to the i th shifted version.
The filter kernel (h(i)) can be selected, as described hereinafter, to provide image smoothing, i.e., reduce noise and image artifacts, or to increase image xe2x80x9csharpnessxe2x80x9d. The modified weighting factor is thus a shifted and weighted average version of the original weighting factor.
By modifying the weighting factor as described above, the reduced noise and artifacts, or increased image xe2x80x9csharpnessxe2x80x9d, in a single slice helical image reconstruction may be achieved. Such algorithm also does not significantly increase the processing time and offers reasonable trade-offs between artifact reduction and slice profile.